In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of current through the gas. One class of HID lamps comprises electrodeless lamps which generate an arc discharge by generating a solenoidal electric field in a high-pressure gaseous lamp fill. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, resulting in a toroidal arc discharge in the arc tube.
At room temperature, the solenoidal electric field produced by the excitation coil is typically not high enough to ionize the gaseous fill and thus initiate the arc discharge. One way to overcome this shortcoming is to lower the gas pressure of the fill, for example, by first immersing the arc tube in liquid nitrogen so that the gas temperature is decreased to a very low value and then allowing the gas temperature to increase. As the temperature rises, an optimum gas density is eventually reached for ionization, or breakdown, of the fill to occur so that an arc discharge is initiated. However, the liquid nitrogen method of initiating an arc discharge is not practical for widespread commercial use.
A recently developed starting aid for an electrodeless HID lamp is a gas probe starter, such as that described in commonly assigned, U.S. Pat. No. 5,095,249 of V. D. Roberts et al., issued of Mar. 10, 1992, which is incorporated by reference herein. The gas probe starter of the Roberts et al. patent includes a fixed starting electrode coupled to a starting chamber which is attached to the arc tube and contains a gas. Preferably, the gas in the starting chamber is at a relatively low pressure as compared with that of the arc tube fill. In the chamber, the gas may be switched between conducting and nonconducting states corresponding to lamp-starting and normal running operation, respectively. In particular, during lamp-starting, a starting voltage is applied to the starting electrode, which causes the gas in the chamber to become conductive. As a result, a sufficiently high voltage is capacitively coupled to the inside surface of the arc tube to break down the gaseous fill contained therein, thus initiating an arc discharge. After the lamp has started, the starting voltage is removed from the starting electrode in order to extinguish the relatively low discharge current in the chamber. In this way, leakage currents flowing between the starting electrode and the arc tube are avoided, thereby extending the useful life of the lamp.
In gas probe starters such as those described hereinabove, gaseous impurities are desorbed from the inside walls of the starting chamber. Such gaseous impurities contaminate the inert gas fill in the chamber and thus increase the voltage required to initiate a gas discharge therein. Hence, it is desirable to provide means for removing the gaseous impurities from the starting chamber fill, thereby decreasing the voltage needed to start the lamp.